"We have received reports of only a small number of targeted attacks and are working to develop a security update to address this issue," said Yunsun Wee, director of Microsoft's Trustworthy Computing group, in a post to the Microsoft Security Response Center blog.

All but one supported edition of IE are affected: 2001's IE6, 2006's IE7, 2009's IE8 and last year's IE9. Together, those browsers accounted for 53 percent of all browsers used worldwide in August. The only exception was IE10, the browser bundled with the new Windows 8, which does not contain the bug.

Monday's advisory was expected, said Andrew Storms, director of security operations at nCircle Security. "I think they had to get it out today," said Storms late Monday in an interview over instant messaging. "Too many people watching and waiting for something official."

Earlier Monday, Microsoft acknowledged that it was investigating reports of a vulnerability but did not promise a patch.

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The bug, when Microsoft gets around to patching it, will be rated "critical," the company's highest threat ranking. Exploiting the flaw allows hackers to execute code - in other words, plant malware on a machine - and opens Windows XP, Vista and Windows 7 to drive-by attacks that only require getting victims to visit a malicious or compromised website.

Until a patch is available, Microsoft recommended that users block attacks with EMET 3.0 (Exploit Mitigation Experience Toolkit), boosting IE's security zone settings to "high," and configuring the browser to display a warning before executing scripts.

EMET is a tool designed for advanced users, primarily enterprise IT professionals, that manually enables anti-exploit technologies such as ASLR (address space layout randomisation) and DEP (data execution prevention) for specific applications.

But not everyone agreed that EMET was the answer.

"[EMET] has its place, but I think most people would prefer the bug fix," said Storms. "EMET is one of those tools that takes time to deploy, [so] it's not a good idea to try and rush the deployment right now. It's kind of like a self-defeating process. Microsoft would like more people to use EMET, but given the few zero-days and relative quickness to patch things, the need for EMET seems to be reduced."

Microsoft may have committed to patching the IE vulnerability, but it has not said whether it will ship an "out-of-band" update, or one outside the regular monthly schedule known as Patch Tuesday.

The next Patch Tuesday is 9 October, three weeks from today.

Storms gave the odds of an out-of-band update a "decent likelihood," but added some caveats. "As usual, the code change is probably the quick part. It's the testing requirements that will take time. Let's see them put their new IE testing resources to work," said Storms.

Storms' reference to resources was a nod to Microsoft's July announcement that it was ditching its longstanding every-other-month patch plan for IE. "We have ... increased our Internet Explorer resources to the point where we will be able to release an update during any month instead of on our previous, bi-monthly cadence," Wee said at the time.

Microsoft will be more likely to release an emergency update if attacks increase or if it cannot come up with an easier way to defend IE than EMET. "If they can deliver a Fixit, they will," said Storms, talking about the automated tools that Microsoft often crafts to configure software settings. "That would [relieve] some of the pressure for a quick patch. If they can't do a Fixit and if the attacks go high, the out-of-band is sure to follow."

An alternative, others have argued, is to stop using IE until Microsoft fixes the bug. Earlier Monday, Rapid7's chief security officer HD Moore advised people to switch, if only temporarily, to Google's Chrome or Mozilla's Firefox.

"I was hoping for easier and less-obtrusive mitigations," Storms said. "My sense is Microsoft is working some late hours to get this [patch] out in a jiffy."

EMET 3.0, the tool Microsoft suggested users deploy, can be downloaded from the company's website. More information about EMET can be found in this support document.